Expanded molded articles comprising a polystyrene-based resin are diversely used as packaging materials and thermal insulating materials since they have superior cushioning properties and thermal insulating properties, as well as are readily molded. However, since impact resistance and resilience are insufficient, and thus cracks and chips readily form, there is the problem that such are not applicable to the packaging and the like of, for example, precision equipment products.
On the other hand, although expanded molded articles comprising a polypropylene-based resin are expanded molded articles having superior impact resistance and resilience, heavy equipment and facilities are required at the time of molding such. Also, due to the nature of the resin, such must be transported from the raw material manufacturer to the molder in the form of expanded particles, which becomes transportation of that having a high volume, thus causing the problem of increased production costs.
In recent years, rubber-modified styrene-based resin expanded molded articles, that is, expanded molded articles made of a high impact polystyrene resin (hereinafter, also referred to as “HIPS”) in which an elastic component such as butadiene rubber is formulated in a polystyrene-based resin, having improved impact resistance and resilience than expanded articles comprising a polystyrene-based resin, as well as being readily molded, have been suggested (for example, refer to Japanese Unexamined Patent Application, First Publication No. S56-67344 (Patent Document 1), Japanese Patent No. 2841303 (Patent Document 2), Japanese Patent No. 4101379 (Patent Document 3), and Japanese Unexamined Patent Application, First Publication No. H03-182529 (Patent Document 4).
For example, in Patent Document 1, expanded particles having improved impact resistance comprising a resin in which non-oriented rubber particles are dispersed in polystyrene are disclosed.
However, since the rubber particles are non-oriented, there are the problems that it is difficult for deformation of rubber particles to occur; in the thin cell membrane forming an expanded article, it is easy for rubber particles to break-through from the cell membrane surface; and, particularly in pre-expanded particles having a highly expanded ratio, retention of the blowing agent gas becomes insufficient by breaking-through of the rubber particles.
Also, in Patent Document 2, an expanded article of a resin formed by mechanically mixing HIPS and a hydrogenated styrene-butadiene block copolymer is disclosed.
However, since a mechanically-mixed rubber component is included in this expanded article, when dispersion of the mixed rubber component is insufficient, there is the problem that dispersion of the rubber component in the expanded article cell membrane becomes unhomogeneous and it is easy for cells to become in communication with each other (production of continuous cells). This tendency is particularly remarkable when expanded at a highly expanded ratio, and the expansion force of expanded particles having a highly expanded ratio decreases to thereby generate voids between particles in the expanded molded article, and thus the appearance of the expanded molded article deteriorates. Also, although the impact resistance of this expanded molded article is improved compared to conventional polystyrene-based expanded molded articles, it is still at a level insufficient for actual use.
Also, Japanese Patent No. 3462775 (Patent Document 5) discloses an expanded molded article of a rubber-modified styrene-based resin composition in which particles of a diene-based rubber encapsulating a polystyrene-based resin are dispersed in a continuous phase comprising a polystyrene-based resin.